Research Article
Effects of dietary inulin on bacterial growth, short-chain fatty acid production and hepatic lipid metabolism in gnotobiotic mice

https://doi.org/10.1016/j.jnutbio.2015.03.010Get rights and content

Abstract

In literature, contradictory effects of dietary fibers and their fermentation products, short-chain fatty acids (SCFA), are described: On one hand, they increase satiety, but on the other hand, they provide additional energy and promote obesity development. We aimed to answer this paradox by investigating the effects of fermentable and non-fermentable fibers on obesity induced by high-fat diet in gnotobiotic C3H/HeOuJ mice colonized with a simplified human microbiota. Mice were fed a high-fat diet supplemented either with 10% cellulose (non-fermentable) or inulin (fermentable) for 6 weeks. Feeding the inulin diet resulted in an increased diet digestibility and reduced feces energy, compared to the cellulose diet with no differences in food intake, suggesting an increased intestinal energy extraction from inulin. However, we observed no increase in body fat/weight. The additional energy provided by the inulin diet led to an increased bacterial proliferation in this group. Supplementation of inulin resulted further in significantly elevated concentrations of total SCFA in cecum and portal vein plasma, with a reduced cecal acetate:propionate ratio. Hepatic expression of genes involved in lipogenesis (Fasn, Gpam) and fatty acid elongation/desaturation (Scd1, Elovl3, Elovl6, Elovl5, Fads1 and Fads2) were decreased in inulin-fed animals. Accordingly, plasma and liver phospholipid composition were changed between the different feeding groups. Concentrations of omega-3 and odd-chain fatty acids were increased in inulin-fed mice, whereas omega-6 fatty acids were reduced. Taken together, these data indicate that, during this short-term feeding, inulin has mainly positive effects on the lipid metabolism, which could cause beneficial effects during obesity development in long-term studies.

Introduction

The prevalence of obesity and obesity-associated diseases is increasing worldwide. Besides other factors, it has been recognized that the intestinal microbiota plays an important role in the development of obesity and related disorders. Studies indicate that microbiota composition between lean and obese individuals differs and that these differences are connected to an increased energy extraction from diet [1], [2]. One important function of the intestinal microbiota is the breakdown of indigestible dietary fiber, which is not completely hydrolyzed by host enzymes in the small intestine. End products from this breakdown are short-chain fatty acids (SCFA), primarily acetate, propionate and butyrate. They provide energy and also act as metabolic regulators. Different dietary fibers lead to the formation of different amounts and specific profiles of SCFA with different metabolic effects. Acetate can be converted to acetyl-CoA and thereby plays an important role in cholesterol biosynthesis and lipogenesis [3], [4]. In contrast, propionate inhibits the uptake of acetate and depresses fatty acid synthesis in hepatocytes. This situation occurs after ingestion of diets rich in fermentable fibers, which are associated with high concentrations of acetate and propionate in the portal vein [5], [6]. Furthermore, propionate is hypothesized to act as a gluconeogenic substrate [7]. This clearly shows that the role of SCFA in energy metabolism has not yet been completely elucidated. Since the type of dietary fiber influences the amount and ratio of the formed SCFA, different dietary fibers can have various effects on energy metabolism. It has been shown that a long-term intervention of a high-fat diet with 10% of fermentable guar fiber leads to a significantly higher body weight gain compared to non-fermentable oat fiber supplementation [8]. This indicates that a higher provision of energy through the supply of fermentable substrates has an obesity-promoting effect. Consistent with this, several studies revealed that obese individuals have significantly higher fecal SCFA concentrations than lean individuals. Since this was not due to changes in the colonic absorption rate or dietary intake, it is taken as evidence for a higher production of SCFA in overweight individuals [9], [10]. In apparent contrast to that, other results show a negative correlation between cecal SCFA production and obesity, and they suggest a protective effect of SCFA against diet-induced weight gain [11], [12]. Fermentable fibers and SCFA are also proposed to increase satiety and counteract obesity by influencing intestinal hormone secretion, liver and adipose tissue function. For example, soluble and fermentable dietary fibers, such as fructans, are considered to reduce body weight gain induced by high-fat diet and fat mass accumulation via attenuation of G-protein-coupled receptor 43 (Gpr43/Ffar2) in white adipose tissue [13]. When summarizing the effects of dietary fiber and SCFA, it is not yet clear if they lead to the prevention or promotion of obesity. It seems that the period of intervention as well as other factors can play an important role in determining the physiological effects.

To identify other dietary fiber-related factors that contribute to the development of obesity induced by high-fat diet, we performed a dietary intervention study in which we compared a non-fermentable vs. a fermentable dietary fiber. To reduce the complexity and the high interindividual variability of the gut microbiota, we used gnotobiotic mice associated with a simplified human intestinal microbiota (SIHUMI) composed of eight bacterial species (Anaerostipes caccae, Bacteroides thetaiotaomicron, Bifidobacterium longum, Blautia producta, Clostridium butyricum, Clostridium ramosum, Escherichia coli and Lactobacillus plantarum) [14]. With this model, we were able to determine microbial changes induced by different dietary fibers and to study the effects of their conversion products on obesity development. We assume that the supplementation of a high-fat diet with 10% of non-fermentable cellulose vs. fermentable inulin leads to differences in the amount of SCFA and influences microbial composition in various ways and thereby affects the profile of produced SCFA. Hence, lipid metabolism can be affected in liver that may play an important role in promotion or prevention of obesity and explain the different physiological effects of dietary fibers.

Section snippets

Animals

Germ-free male C3H/HeOuJ mice were purchased from Charles River. Germ-free mice (11–13 weeks old) were associated with a SIHUMI consisting of eight bacterial species (A. caccae, B. thetaiotaomicron, B. longum, B. producta, C. ramosum, C. butyricum, E. coli and L. plantarum) and adapted for 2 weeks to the consortium. Subsequently colonization was verified by using quantitative real-time polymerase chain reaction (qRT-PCR). Individually housed mice were maintained in positive-pressure isolators

Inulin alters energy homeostasis without affecting body weight composition

In week 4 of intervention, feed intake as well as dietary energy intake were not different between HFC and HFI animals (Table 2). In contrast, feces energy of the HFI group was reduced compared to mice fed a non-fermentable HFC diet. This was due a large increase in daily fecal bulk in the cellulose-fed group. Therefore, diet digestibility was significantly different between the groups with an about 10% higher digestibility (95%) in the HFI compared to the HFC group as expected. However, the

Discussion

In the present study, we compared the short-term effects of a fermentable fiber (inulin) and a non-fermentable fiber (cellulose) on energy homeostasis, intestinal microbiota and lipid metabolism to clarify the role of dietary fibers in obesity development. We show that inulin enhanced bacterial growth because it served as an additional bacterial substrate. In accordance, total SCFA production was increased, accompanied by changes in the acetate:propionate ratio. Inulin further induced

Acknowledgements

Inulin (Fibruline DS2) was kindly provided from Georg Breuer GmbH (Königstein, Germany). We gratefully thank Carolin Borchert and Antje Sylvester for technical assistance and Andreas Wernitz for lipid analysis. We thank furthermore Ines Grüner and Ute Lehmann for taking care of the animals. This work was supported by funding to G.L. from the Danone Institute and S.K. from the Deutsche Forschungsgemeinschaft, Bonn, Germany (KL613/18-1). All authors read and approved the final manuscript. The

References (47)

  • E.J. Masoro et al.

    Propionic acid as a precursor in the biosynthesis of animal fatty acids

    J Lipid Res

    (1961)
  • P.M. Nishina et al.

    Effects of propionate on lipid biosynthesis in isolated rat hepatocytes

    J Nutr

    (1990)
  • N.M. Delzenne et al.

    Biochemical basis of oligofructose-induced hypolipidemia in animal models

    J Nutr

    (1999)
  • D. Letexier et al.

    Addition of inulin to a moderately high-carbohydrate diet reduces hepatic lipogenesis and plasma triacylglycerol concentrations in humans

    Am J Clin Nutr

    (2003)
  • U.P. Kelavkar et al.

    Prostate tumor growth and recurrence can be modulated by the omega-6:omega-3 ratio in diet: athymic mouse xenograft model simulating radical prostatectomy

    Neoplasia

    (2006)
  • S.A. Bingham et al.

    Dietary fibre in food and protection against colorectal cancer in the European Prospective Investigation into Cancer and Nutrition (EPIC): an observational study

    Lancet

    (2003)
  • U.A. Ajani et al.

    Dietary fiber and C-reactive protein: findings from national health and nutrition examination survey data

    J Nutr

    (2004)
  • P.J. Turnbaugh et al.

    An obesity-associated gut microbiome with increased capacity for energy harvest

    Nature

    (2006)
  • R.W. Hanson et al.

    The relative significance of acetate and glucose as precursors for lipid synthesis in liver and adipose tissue from ruminants

    Biochem J

    (1967)
  • C. Demigne et al.

    Effect of propionate on fatty acid and cholesterol synthesis and on acetate metabolism in isolated rat hepatocytes

    Br J Nutr

    (1995)
  • A. Verbrugghe et al.

    Propionate absorbed from the colon acts as gluconeogenic substrate in a strict carnivore, the domestic cat (Felis catus)

    J Anim Physiol Anim Nutr (Berl)

    (2012)
  • S. Rahat-Rozenbloom et al.

    Evidence for greater production of colonic short-chain fatty acids in overweight than lean humans

    Int J Obes (Lond)

    (2014)
  • A. Schwiertz et al.

    Microbiota and SCFA in lean and overweight healthy subjects

    Obesity (Silver Spring)

    (2010)
  • Cited by (0)

    Funding: This work was supported by grants to G.L. from the Danone Institute and to S.K. from the Deutsche Forschungsgemeinschaft (KL613/18-1). The funding bodies had no involvement in study design; collection, analysis and interpretation of data; writing of the article; and decision to submit the article for publication.

    1

    These authors contributed equally.

    View full text